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SIDEROPHORE DESFERRIOXAMINE B PROMOTED DISSOLUTION AND ADSORPTION PROPERTIES OF TITANIUM DIOXIDE BEARING MINERALS
Haque, Md Imdadul
Haque, Md Imdadul
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2024-08
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Chemistry
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http://dx.doi.org/10.34944/dspace/10596
Abstract
Titanium dioxide nanoparticles (TiO2-NPs) are prevalent in products of everyday use, especially in food additives, paints, medical implants, and consumer products. Their favorable properties include inertness in an aqueous environment. Even though TiO2 is not considered potentially toxic to humans as it is chemically stable and not soluble in an aqueous environment, it still can play an important role in many biological processes. Therefore, it is of great importance to figure out their potential dissolution and transformation in biorelevant conditions.Transition metals like iron are considered an essential element for many microorganisms and especially for marine and terrestrial systems. However, their bioavailability is restricted due to the low solubility of Fe(III) in aqueous environment. In that scenario, small biomolecules known as microbial siderophores which are produced by many microorganisms such as yeast, fungi, algae, and bacteria in a low iron available environment play a significant role in iron availability. They can be found everywhere in nature and can sequester Fe(III) in a low Fe environment and make them soluble which can be further accessible by marine and terrestrial microorganisms. Numerous studies have been conducted focusing on the bioavailability of Fe by microbial siderophores. However, the bioavailability of Ti is largely overlooked due to its nonessential role in humans. Ti(IV) is chemically similar to Fe(III) and it is an even stronger Lewis acid than Fe(III). Therefore, it can interact strongly with bacterial siderophores by acid-base chemistry.
Given the importance of TiO2 nanoparticles in everyday life and their unexplored biological roles, this research explores the dissolution of different crystalline polymorphs of TiO2 such as anatase and rutile by siderophore desferrioxamine B (DFOB) in environmental conditions. Spectroscopic and microscopic techniques were used to understand their bioavailable nature at different pH. Dissolution of Ti from the surface of anatase and rutile nanoparticles was found pH dependent. UV/Vis spectroscopy revealed different Ti-DFOB complex formations at different pH and their solubilized Ti was quantified from Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). This study suggests that as high as ~ 30 μM dissolved Ti can be found at low pH by DFOB whereas control experiments without DFOB suggest only a nanomolar level of dissolved Ti. Kinetic data confirmed that several complex steps were associated with anatase (three to four steps) and rutile (two steps) respectively. Rutile nanoparticles exhibited promising dissolution when considering long-term effects and even very low concentrations of DFOB (< 200 μM).
The surface study of DFOB-treated TiO2 from scanning electron microscopic (SEM) and transmission electron microscope (TEM) revealed possible surface changes from erosion and formation of smaller nanoparticles. Electrospray Ionization Mass Spectroscopy (ESI-MS) showed the formation of Ti(IV)-DFOB complex in solution from the outcome of dissolution. The effect of mixed ligand study was explored considering the complex matrix environment by introducing small organic acids such as oxalate, citrate, humic acid, and ascorbate. They exhibited mostly synergistic effects and increased the amount of dissolved Ti except for humic acid. Understanding the dissolution of TiO2 in different environmental conditions is necessary to determine the biological effects in water bodies.
Attenuated Total reflectance - Fourier Transform Infrared Spectroscopy (ATR-FTIR) analysis was conducted to determine the vibrational modes associated when DFOB interacting with TiO2. This study indicated that two to three hydroxamic acids were covalently bonded to the surface of TiO2 by inner-sphere coordination. Adsorption was also found to be pH-dependent. Dissolution vs adsorption study suggested that dissolution is more favorable at pH 3 whereas adsorption was more favorable at pH 6.
In addition to the anatase and rutile nanoparticles, a commercial food-grade TiO2 was also investigated given the importance in human health. Dissolution study revealed the highest amount of dissolved Ti (~ 60 μM) under similar conditions when compared to anatase and rutile. Physiochemical characterization such as particle sizes, zeta potential, and any possible surface coating elements was measured to understand the higher reactivity of food-grade TiO2. The isoelectric point of different forms of TiO2 was found to be different and indicated their different behavior in terms of reactivity.
Finally, given the environmental implications of titanium, a standard reference material of coal fly ash (CFA) sample which is a fine powdered material primarily composed of silica, alumina, iron oxide, and calcium oxide produced from coal combustion and contains Ti as a trace amount in its complex mixture was investigated by DFOB and humic acid. CFA could be a major source of several elements in the atmosphere and their deposition in water bodies could be affected by DFOB.
Overall, the demonstration that DFOB can solubilize Ti from different nanosized crystalline polymorphs, food-grade TiO2, and complex matrix of coal fly ash has an environmental and health significance. Understanding this work could be beneficial for the application in the food-processing industry, the potential toxicity study of Ti, and how Ti behaves chemically in biological systems.
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